I got this working in the end by changing my functions php to:
// Make sure featured images are enabled
add_theme_support( 'post-thumbnails' );
// Add featured image sizes
add_image_size( 'home_portrait', 1000, 1000, false ); // width, height, crop
// Register the three useful image sizes for use in Add Media modal
add_filter( 'image_size_names_choose', 'wpshout_custom_sizes' );
function wpshout_custom_sizes( $sizes ) {
return array_merge( $sizes, array(
'home_portrait' => __( 'Home Portrait' ),
) );
Though I am still unsure why the initial code did not work
